Aluminum-resistant refractory material



United States Patent 3,067,146 AtUMrNUM-nEsrsTANr nEFRAcronY MATERIAL Gottfried A. Rubin, Potsdam, N.Y., assignor to Union Carbide Corporation, a corporation of New York No Drawing. Filed Jan. 17,1961, Ser. No. 83,139

6 Claims. (Cl. 252-620) This invention is concerned with a novel aluminumresistant refractory material which is particularly useiul as a shot cylinder in hot chamber aluminum die casting. More specifically, this invention relates to a novel composition containing alumina, titanium diboride and chromium oxide.

The main object of this invention is to provide a material which is more oxidation resistant than metal-bonded hard metals, is less brittle than pure oxides and has great strength at elevated temperatures.

Another object of this invention is to provide an aluminum-resistant composition.

A further object of the invention is to provide a novel composition containing alumina, titanium diboride and chromium oxide, the electrical conductivity of which can be variedby varying the composition and grain size of the constituents.

The composition by means of which the foregoing objects are attained comprises from to 80 percent by weight of titanium diboride, the balance being aluminum oxide containing from about 1 to about 3 percent of chromic oxide (Cr O Electrical conductivity of the finished article can be varied by varying the titanium diboride within the indicated range.

Compositions containing the above named constituents, may be shaped by hot pressing or by isostatic pressing, extrusion, by slip casting or by spin casting of the blended powders, followed by sintering.

As an example of the practice of this invention, various shapes were made by hot pressing at temperatures between 1600" C. and 1900 C. from a composition indicated below. These shapes had properties also listed below.

Composition (by weight):

ohm-cm In another example of the practice of the invention, the feasibility of vibrational packing and sintering operations for the fabrication of die casting machine cylinders was successfully demonstrated. For this example, a graphite crucible was procured having the following dimensions: 4 inches in length; 2.9 inches outside diameter with a 2.0 inches inside diameter and a small cavity 0.75 inch in diameter at the center of the bottom. A hollow paper cylinder having the same diameter and 5 inches in 3,057,140 Patented Dec. 4, 195?.

length was prepared and inserted in the cavity. A blend consisting of 35 percent of titanium diboride, 63.7 percent of alumina, and 1.3 percent of chromic oxide was placed in the hollow cylindrical space between the paper and the crucible. This blend was densely packed in place by mechanical vibration. The crucible, then, was fired to 1800 C. and held at that temperature for one hour. As a result of this heating, the paper crucible was converted to carbon, thereby keeping the center part free of material. The sintered piece, made from the blend, came out as a dense, 3 inches long tube of homogeneous structure without cracks or flaws. Its outer diameter had shrunk from 2.0 inches to 1.65 inches or about 17 percent. The piece was cut along a plane perpendicular to the axis 1 inch below the top. The bulk density of both parts so formed was 2.81 g./cm. showing a porosity of about 30 percent. The fiexural .strength measured at room temperature on a length segment of the tube in a radial direction was about 12,000 p.s.i. This fiexural strength is very high considering the high porosity of the piece.

A variant of the invention employs spin-casting, w ich consists in forming a liquid suspension of the mixture to be sintered containing about 15 to 20 percent by weight of a liquid such as water. The suspension then is poured into a suitable forming vessel such as a cylindrical crucible. The crucible is closed tightly and is rotated at a high circular velocity around the cylinder axis which is maintained in a horizontal position. During this step, the mixture forms a hollow cylinder of uniform diameter and 30 to 40 percent of the liquid phase are separated from the solid phase. crucible is dried. The crucible and its spun mixture are then placed in a furnace for sintering. Using the spincasting method, the mixture disclosed in the above example was processed to form a crucible having a density 94 percent of theoretical and a fiexural strength, in the radial direction, of 20,000 psi. at room temperature.

In another variant, fugitive binders, which do not deposit much carbon such as epoxy resins, vinyl resins and rosin are used with the dry materials. In the examples appearing in the table below, two cylinders were cold prepressed from blends which contained a 5 percent addition by weight thereof of an epoxy resin and then hot-pressed.

Table I Fabrication Cylinder 1 Cylinder 2 Blend:

A1 0 63.7 parts by weight". 63.7 parts by weight.

Cr 0 1.3 parts by weight. 1.3 parts by weight.

"liB:v 35.0 parts by weight-.. 35.0 parts by weight.

Resin 1 5.0 parts by weight 5.0 parts by weight. Cold Pressing:

Pressure 28,000 psi. 44,000 psi.

Bulk Density. 2.83 g./crn. Curing 250 0.; 3 hrs. Hot Pressing:

Temperature 1,800 C 1,800 0.

Hold PeriotL- 1 hour-" 20 minutes.

Pressure 2,000 p s 2,000 p.s.i Final Density 3.97 gjcm. 5. 3.95 g./cm. (05.0% of theoretical). theoretical) 1 %s OD 4% OD. Final Dimensions $4 ID 2 ID.

1 16,000 cp. viscosity epoxy resin to which a 5-10 percent addition of triethanolamine is added as a hardener. A suitable resin is grade 13111.- 3794 made by Union Carbide Corporation.

The cylinders described above, although exhibiting ertremely high hardness, can be diamond machined to very close tolerances, such as are required for die casting shot cylinders.

Compositions containing titanium diboride in the range of from 20 to 95% and having an alumina-chromic oxide content ranging from 5 to have been formed into useful shapes in accordance with the procedures described The free liquid then is removed and the (it above. With increasing amounts of titanium diboride the electrical resistivity of the fired shapes was found to decrease; the range of resistivity, at room temperature varying from 20 to 8000 nohm-cm.

The amount of chromic oxide required for optimum final properties of the composite in terms of high temperature strength, resistance against attack by liquid aluminum and the achieving of high fabricated density is found to be in the range of from 1 to 3% of the alumina content of the composite.

Articles made with the compositions of the invention are especially inert under the environment of hot chamber aluminum die casting. In particular, it was noted that immersion in aluminum for 16 days at 750 3., did not produce attack on these articles.

What is claimed is:

1. Sintered electrically-conductive, aluminunnesistant articles comprising from about 5 to about 80 percent by weight of titanium diboride, the balance being aluminum oxide containing a 1 to 3% addition of cliromic oxide.

2. A sintered electrically conductive aluminum resistant article which comprises about 35.0 parts by weight titanium diboride, about 63.7 parts by weight aluminum oxide, and about 1.3 parts by weight chromic oxide.

3. A method for forming electrically-conductive refractory articles which comprise from about 5 to about 80 percent by weight of titanium diboride, the balance 4 being aluminum oxide containing a l to 3% addition of chromic oxide; said method comprising forming a blend comprising titanium diboride, chromic oxide and alumina in the indicated proportions, shaping said blend and sintering the resulting shape at a temperature between about 1600 C. and 1900 C.

4. A method for forming electrically-conductive refraetory articles which comprise from about 5 to about 80 percent by weight of titanium diboride, the balance being aluminum oxide containing a 1 to 3% addition of chromic oxide; said method comprising forming a blend comprising titanium diboride, chromic oxide, alumina in the indicated proportions and a liquid, inserting said blend in a die, rapidly rotating said die and sintering said blend while the same is in said die.

5. The method of claim 4, wherein said blend contains a fugitive binder.

6. The method of claim 4, wherein said liquid is water present in an amount ranging from about 15 to about 20 percent by weight of said mixture and said water is removed from said blend prior to sintering.

References Cited in the file of this patent UNITED STATES PATENTS 2,874,065 Herz et al. -a Feb. 17, 1959 2,936,505 Witucki et al. May 17, 1.960 2,984,807 Blum May l6, i961 

1. SINTERED ELECTRICALLY-CONDUCTIVE, ALUMINUM-RESISTANT ARTICLES COMPRISING FROM ABOUT 5 TO ABOUT 80 PERCENT BY WEIGHT OF TITANIUM DIBORIDE, THE BALANCE BEING ALUMINUM OXIDE CONTAINING A 1 TO 3% ADDITION OF CHROMIC OXIDE. 